eDRAM having dynamic retention and performance tradeoff

ABSTRACT

A semiconductor chip has an embedded dynamic random access memory (eDRAM) in an independently voltage controlled silicon region that is a circuit element useful for controlling capacitor values of eDRAM deep trench capacitors and threshold voltages of field effect transistors overlying the independently voltage controlled silicon region. Retention time and performance of the eDRAM is controlled by applying a voltage to the independently voltage controlled silicon region.

FIELD OF THE INVENTION

This invention relates generally to semiconductor chips, and morespecifically to an eDRAM (embedded dynamic random access memory) in anindependently voltage controlled volume of silicon on an SOI (silicon oninsulator) semiconductor chip.

SUMMARY OF EMBODIMENTS OF THE INVENTION

An SOI chip has a substrate that is typically P− doped silicon, althoughsubstrates of opposite doping (i.e., N−) are also known. A buried oxide(BOX) layer may be implanted to isolate a circuit area above the BOXlayer from the underlying substrate portion. The underlying substrateportion is typically connected to a voltage source (e.g., Gnd). Abovethe BOX, the circuit area may contain STI (shallow trench isolation)regions, source/drain implants for FETs (Field Effect Transistors), bodyregions under FET gate structures for the FETs, contacts, and wiring tointerconnect the FETs.

In an embodiment of the invention, an eDRAM is formed in anindependently voltage controlled silicon region which is created as acircuit element. A bottom of the independently voltage controlledsilicon region is created with a deep implant such as boron to create anN region when the substrate is doped P−. Sides of the independentlyvoltage controlled silicon region are formed with deep trench isolation,thereby insulating the independently voltage controlled silicon regionon all sides (e.g., four sides if the independently voltage controlledsilicon region is square or rectangular). A buried oxide region (BOX)forms a top surface of the independently voltage controlled siliconregion, thereby completing electrical isolation of the independentlyvoltage controlled silicon region. An electrical contact is formedthrough the BOX, and through any STI or silicon above the BOX, theelectrical contact suitable for connecting the independently voltagecontrolled silicon region to a voltage such as Vdd, Gnd, or othervoltage supply, or to a logic signal on the chip.

A voltage, such as from a voltage source or from a logic signal, may beplaced on the independently voltage controlled silicon region via thecontact. The voltage creates an electric field that passes through theBOX and determines a threshold voltage on a pass gate FET which is usedto place charge in a deep trench capacitor used to store a “1” or a “0”in the eDRAM. The voltage also affects width of a charge depletionregion around a portion of the eDRAM deep trench capacitor that is inthe independently voltage controlled silicon region. Control of leakagethrough the pass gate FET and width of the charge depletion region bythe voltage provides tradeoffs between retention time of the eDRAM andperformance of the eDRAM.

While for exemplary purposes, eDRAM is described, it will be understoodthat, if all or a majority of a semiconductor chip comprises theteachings described herein, that the semiconductor chip may be simplycalled a DRAM (dynamic random access memory).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of a portion of a semiconductor chip, showing alogic region and an eDRAM region, the eDRAM region including anindependently voltage controlled volume of silicon.

FIGS. 2A-2E show key process steps in creation of an independentlyvoltage controlled volume of silicon.

FIG. 3 shows a cross section of a semiconductor chip having twoindependently voltage controlled volumes, each containing an eDRAM cell.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description of embodiments of the invention,reference is made to the accompanying drawings, which form a parthereof, and within which are shown by way of illustration specificembodiments by which the invention may be practiced. It is to beunderstood that other embodiments may be utilized and structural changesmay be made without departing from the scope of the invention.

Embodiments of the present invention provide for creation of anindependently voltage controlled volume of silicon which is a circuitelement generally useful for providing selectable control ofleakage/performance characteristics of an eDRAM (embedded dynamic randomaccess memory) on a silicon chip.

A semiconductor silicon on insulator (SOI) chip 100 of FIG. 1 is shownhaving a logic area 150 and an eDRAM area 151.

Logic area 150 comprises a portion of P− Silicon 101, which is typicallyconnected to ground. A buried oxide (BOX) 103 provides an electricinsulator under a logic FET (field effect transistor) 120. Logic FET 120includes source/drain implants 121, a P− body region 125, a gatedielectric 126, source/drain contacts 122, gate sidewall spacers 123,and a gate 124 that may be electrically coupled to a logic signal or avoltage source. Logic FET 120, having a P− body and N+ source/drainregions is an NFET (N-channel field effect transistor). Typically, PFETs(P-channel field effect transistors) are also created in logic area 150using known techniques to create an N− body region and P+ source/drainregions. The NFETs and PFETs in logic region 150 are configured to makelogic gates (NANDs, NORs, XORs, latches, registers, and the like).

eDRAM area 151 comprises a pass gate NFET 130 to couple a bit lineconnected to a source/drain implant 131 to a deep trench capacitor 140under control of a word line coupled to a gate 134. Pass gate NFET 130includes the gate 134, a gate dielectric 136, source/drain implants 131and 132, a body 135, a gate dielectric 136, sidewall spacers 133, andepitaxial growths 137 and 138. Deep trench capacitor 140 comprises aconductor 141 in a deep trench. The conductor may be tungsten, dopedpolysilicon, or other suitable conducting material placed in the deeptrench. A dielectric material 142 isolates conductor 141 from P− silicon101 and P− silicon 109. Dielectric material 142 may be, for examplesHfO2 or SiO2, or other suitable dielectric material. Epitaxial growth137 couples an adjacent source/drain region 132 over an upper portion ofdielectric material 142 to make electrical contact between conductor 141and the adjacent source/drain region 132.

eDRAM area 151 also comprises deep N implant 105, which forms a “floor”,or bottom, of independently voltage controlled silicon region 110,indicated by a dotted line in FIG. 1. N implant 105 may be a deep boronimplant of high enough energy to create N implant 105 at a depth insemiconductor chip 100 that is less deep than deep trench isolation 106,as depicted in FIG. 1, but deep enough to include most or all of aportion of deep trench capacitor 140 below BOX 103. For example, over50% of deep trench capacitor 140 should face P− Si 109. Note that deeptrench capacitor 140 need not extend to N implant 105. A 4 MeV (millionelectron volts) boron implant will have a peak dose at about 20 um; a 2MeV boron implant will have a peak dose at about 10 um.

A “ceiling”, or top, of the independently voltage controlled siliconregion 110 is a portion of BOX 103. Sides of the independently voltagecontrolled silicon region 110 are formed by a deep trench isolation 106,best seen in FIG. 2E in a top view. N implant 105 must be wide enough toensure that P− silicon 109 is not in electrical contact with P− silicon101.

A contacting structure 107 is formed by etching through STI (shallowtrench isolation) 102 and through BOX 103 and filled with a conductorsuch as tungsten or doped polysilicon to make electrical connection toP− Si 109. Contacting structure 107 may have a contact 108 to connect toa voltage (voltage source or a logic signal). Except for contactingstructure 107, P− silicon 109 is completely isolated, as describedabove, from P− silicon 101 and circuitry (e.g., pass gate NFET 130)above BOX 103. Contacting structure 107 transfers the voltage placed oncontact 108 to P− silicon 109, thereby providing a voltage onindependently voltage controlled silicon region 110.

A single NFET pass gate 130 and an associated deep trench capacitor 140is shown in eDRAM area 151, however it will be appreciated that a largenumber, perhaps one million or more, NFET pass gates 130 and associatedcapacitors 140 are typically placed in an eDRAM area 151. Similarly, forsimplicity, a single LOGIC FET 120 is shown in logic area 150. However,in modern semiconductor chips 100, one million, or more, FETs 120 may beconstructed.

It will also be appreciated that, while NFET pass gate 130 is shown as aswitch to charge or discharge deep trench capacitor 140, and to, onreads, cause a charge on deep trench capacitor 140 to affect a bit linevoltage, a PFET, with known processing above BOX 103 could also be usedas a pass gate.

With reference now to FIGS. 2A-2E, a series of key processing steps isshown to create independently voltage controlled silicon region 110. InFIG. 2A, semiconductor chip 100 receives high energy boron implant 301through a mask 302, thereby creating N implant 105 at a depth determinedby implant energy and semiconductor structure. As noted above, a 4 MeVboron implant will create a N implant 105 approximately 20 um below atop surface of semiconductor chip 100.

FIG. 2B shows a conventional oxygen implant 303 applied to semiconductorchip 100 to create BOX 103 at a depth determined by energy of the oxygenimplant 302.

FIG. 2C shows creation of a deep trench isolation 106 that extends atleast down to, and advantageously slightly below, N implant 105. Deeptrench isolation may be created using a conventional process such asused to create eDRAM capacitors, but is elongated to form sides of theindependently voltage controlled silicon region 110. Alternatively, deeptrench isolation 106 may utilize a deep trench capacitor structure astaught in copending application US 2011/0018094, also assigned to thepresent assignee. Following construction of deep trench isolation 106,BOX 103, and N implant 105, P− Si 109 is totally isolated electrically.P− Si 109 is merely an electrically isolated portion of P− Si 101 anddoes not receive a separate implant.

FIGS. 2D and 2E show, respectively, a cross sectional (through AA) viewand a top view of a portion of semiconductor chip 100 generally in thearea where independently voltage controlled silicon region 110 isconstructed. Shallow trench isolation (STI) 102 is formed in silicon 111(i.e., the portion of P− Si 101 above BOX 103 as shown in the finelycrosshatched portions with crosshatching running up and to the left.Contact structure 107 is created by an oxide etch through STI 102 andBOX 103. A contact 108 may be formed atop contact structure 107. FIG. 2Eshows a top view of that portion of semiconductor chip 100. NFET passgate 130 (FIG. 1) is formed by conventional means in silicon 111 in aconventional manner, creating source/drain implants 131, 132, creationof gate dielectric 136, creation of spacers 133, epitaxial growth 137and 138 after etching, lining, and filling of deep trench capacitor 140.

FIG. 3 shows two independently voltage controlled silicon regions 110,referenced 110A (left instance) and 110B (right instance), with 110A and110B sharing a common deep trench isolation 106 portion between them,for simplicity of illustration. Key referenced items have an “A” suffix(e.g., 130A for the left hand NFET pass gate 130) for referenced itemsassociated with independently voltage controlled silicon region 110A,and a “B” suffix for referenced items associated with independentlyvoltage controlled silicon region 110B.

In FIG. 3, VA, attached to contact structure 107A with contact 108A mayhave a voltage of 0.0 volts, thereby causing P− Si 109A to be at 0.0volts. VB, attached to contact structure 107B with contact 108B may havea voltage of +5.0 volts, thereby causing P− Si 109B to be at 5.0 volts.Width of charge depletion regions 144 (144A, 144B) around deep trenchcapacitor 140 (140A, 140B) is dependent on voltage between a voltage onconductor 141 (141A, 141B) and a voltage applied to P− Si 109 (109A,109B). To a first order, separation of capacitor plates of capacitor C(CA, CB) correspond to width of the charge depletion region. It will beunderstood that deep trench capacitor 140 is schematically shown ascapacitor C. Deep trench capacitor 140A is shown schematically as CA;deep trench capacitor 140B is shown schematically as capacitor CB. Ifthe charge depletion region 144 (144A, 144B in FIG. 3) is wider, thecapacitor plates are further apart, and the capacitance is less. Usingthe VA, VB voltages assumed, independently voltage controlled siliconregion 110A will have a wider charge depletion region 144A around deeptrench capacitor 140A than a width of charge depletion region 144B inindependently voltage controlled silicon region 110B around deep trenchcapacitor 140B. Therefore, CA is shown has having capacitor platesfurther apart than CB. CA will have less capacitance than CB.

Another effect of the voltage (VA, VB) placed on P− Si 109A, 109B isthat an electric field 302 (302A, 302B) passes through BOX 103 andaffects threshold voltages of overlying FETs, such as NFET pass gates130A, 130B. As shown, with the assumed values of VA, VB, electric field302A is less than electric field 302B.

In terms of controlling characteristics of eDRAM cells in P− Si 109A,threshold voltage of NFET pass gate 130A will be higher than a thresholdvoltage of NFET pass gate 130B, thereby significantly lowering leakageof NFET pass gate 130A relative to NFET pass gate 130B. Capacitance ofCA, as explained earlier is less than CB, but significantly reducedleakage from CA through NFET pass gate 130A versus leakage from CBthrough NFET pass gate 130B will cause retention of data in deep trenchcapacitor 140A (i.e., CA) to be longer than retention of data in deeptrench capacitor 140B (i.e., CB) even though CB is a larger capacitance.Therefore, eDRAMs may be controlled to leak more or less by control ofvoltage applied to the associated P− Si 109 in independently voltagecontrolled silicon region 110. This leakage control capability is verydesirable in low power modes of an eDRAM.

For performance, such as read speed, however, the eDRAM in independentlyvoltage controlled silicon region 110B will be superior (faster) versusthe eDRAM in independently voltage controlled silicon region 110A. NFETpass gate 130B, having a lower threshold voltage will conduct morestrongly. Also, the larger capacitance of CB will pull a bit line downfaster and further through NFET pass gate 130B than the lessercapacitance and less conductive structure associated with independentlyvoltage controlled silicon region 110A. Therefore, eDRAMs may becontrolled to operate faster (or slower) by control of the associated P−Si 109 in independently voltage controlled silicon region 110.

Applying the electric field 302 and capacitor C to FIG. 1 which has alogic area 150 as well as an eDRAM area 151, it is clear that a voltageapplied to P− Si 109 is not going to affect a threshold voltage in LOGICFET 120, since LOGIC FET 120 is constructed over P− Si 101, which is atGnd, rather than being constructed over a P− Si 109. It is of coursetrue that P− Si 101 can be connected to a voltage source other thanground, and thereby affect threshold voltage of any FET overlying thatbiased P− 101, however doing so would affect PFETs and NFETs in anopposite manner (for example, PFET strength would decrease when NFETstrength increases) and therefore it would be undesirable to do so.Embodiments of the current invention provide for one or moreindependently voltage controlled silicon regions on a semiconductorchip. PFET/NFET relative strength in eDRAM applications is not an issue,since the eDRAM regions 151 typically contain only NFETs (i.e., NFETpass gates 130).

While for exemplary purposes, eDRAM is described, it will be understoodthat, if all or a majority of a semiconductor chip comprises theteachings described herein to provide dynamic control of retention ofdata in a deep trench capacitor and performance of the DRAM, that thesemiconductor chip may be simply called a DRAM (dynamic random accessmemory) chip, and the memory simply called a DRAM.

What is claimed is:
 1. A semiconductor chip comprising: an independentlyvoltage controlled silicon region further comprising: a deep implant ofopposite doping to a substrate doping of the semiconductor chip, thedeep implant forming a bottom of the independently voltage controlledsilicon region; a buried oxide (BOX) implant forming a top of theindependently voltage controlled silicon region; a deep trench isolationforming sides of the independently voltage controlled silicon region;and a contact structure of electrically conducting material formedthrough the buried oxide to provide electrical contact to theindependently voltage controlled silicon region; an embedded dynamicrandom access memory further comprising a deep trench capacitor, whichextends through the BOX and at least partly through the independentlyvoltage controlled silicon region; a FET pass gate formed above theindependently voltage controlled silicon region for charging the deeptrench capacitor and for sensing a voltage on the deep trench capacitor.2. The semiconductor chip of claim 1, the deep implant of oppositedoping to the substrate doping of the semiconductor chip being a borondeep implant of 2 MeV to 4 MeV energy, the substrate having a P− doping.3. The semiconductor chip of claim 1, wherein the contact structurecouples the independently voltage controlled silicon region to a voltagesupply.
 4. The semiconductor chip of claim 1, wherein the contactstructure couples the independently voltage controlled silicon region toa logic signal.
 5. A method for controlling an eDRAM (embedded dynamicrandom access memory) retention time and performance comprising:creating an independently voltage controlled silicon region in asemiconductor chip comprising: creating a deep implant of oppositedoping to a doping of a substrate of the semiconductor chip; creating aburied oxide (BOX) in the semiconductor chip, the deep implant beingentirely under the BOX; creating a deep trench isolation at least asdeep in the substrate as the deep implant and intersecting both the deepimplant and the BOX; the deep implant forming a bottom of theindependently voltage controlled silicon region; the BOX forming a topof the independently voltage controlled silicon region; the deep trenchisolation forming walls of the independently voltage controlled siliconregion, thereby completely isolating the independently voltagecontrolled silicon region from a remainder of the substrate; andcreating a contact structure to electrically connect the independentlyvoltage controlled silicon region to a contact creating an deep trenchcapacitor that passes through the BOX and at least part way through theindependently voltage controlled silicon region; creating a pass gatetransistor above the independently voltage controlled silicon region forcharging the deep trench capacitor and for sensing a voltage on the deeptrench capacitor.
 6. The method of claim 5 further comprising applying avoltage to the independently voltage controlled silicon region byconnecting the contact to a voltage source.
 7. The method of claim 6wherein the voltage source is a logical signal.
 8. The method of claim6, the voltage applied to the independently voltage controlled siliconregion affecting a threshold voltage of the pass gate transistor.
 9. Themethod of claim 6, the voltage applied to the independently voltagecontrolled silicon region affecting a width of a charge depletion regionof the deep trench capacitor.